1. Field of the Invention
The present invention relates generally to apparatus and methods for recovering energy from geothermal reservoirs. More specifically, the present invention relates to apparatus and methods for retrieving and converting geothermal energy employing at least partially depleted hydrocarbon wells.
2. Description of the Prior Art
As the world continues to focus more and more attention on the effects of greenhouse gas emissions on global warming, greater efforts are being spent searching for cleaner and renewable energy sources. One such source comes in the form of geothermal energy. Often, geothermal energy is obtained by exploitation of geothermal water reservoirs.
One method of obtaining and converting geothermal energy comes from the use of a dry steam power plant. Dry steam power plants use dry steam from geothermal reservoirs produced at the surface to drive a turbine coupled to a generator, thereby generating electricity. Another method involves the use of a flash steam power plant, which uses produced geothermal water under high pressure to generate steam via a flash tank. The steam, in turn, can be used to drive a turbine coupled to a generator to create electricity. A third method involves a binary cycle power plant. In a binary cycle power plant, geothermal water is pumped to the surface and passed through a surface-level heat exchanger, which transfers heat to a working fluid. At least a portion of the working fluid is typically converted to a vapor phase in the heat exchanger and can then be used to power a turbine.
In each of the above processes, a geothermal water reservoir must be found in a subterranean formation that is sufficiently porous and permeable so as to facilitate pumping the geothermal water to surface level for use. Such restrictions severely limit the number of reservoirs that may be used to produce geothermal energy. Techniques do exist to enhance porosity and permeability, but such operations are costly and time consuming. Accordingly, there is a need for geothermal energy production processes that do not require the subterranean formations containing geothermal water reservoirs to be highly permeable and porous.
After a hydrocarbon well has exceeded its economic usefulness, the depleted well is typically filled with cement and abandoned. Many of these hydrocarbon wells have high bottom-hole temperatures, sometimes in excess of 300 or 400° F. Further, many of these wells extend through or are in close proximity to geothermal water reservoirs. Accordingly, many of these abandoned wells contain or are proximate to a potential untapped energy resource. Thus, in addition to the need for improved methods and apparatus for geothermal energy extraction, there is also a need for efficient use of depleted hydrocarbon wells.